Nanocrystalline bolt and method for manufacturing same

ABSTRACT

A nanocrystalline bolt, which serves as a high-strength bolt, is manufactured by the steps of compacting powder formed of nanocrystalline metal particles having a nanometer-scale grain size, extruding a resultant compact so as to obtain a rod stock, and subjecting the rod stock to a bolt-forming process (heading, machining, and form rolling) without involvement of heat treatment. Examples of metal material include JIS SUS410, precipitation-hardening-type 13Cr-8Ni-2Mo, and other stainless steels.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a nanocrystalline bolt, which serves as a high-strength bolt, and a method for manufacturing the nanocrystalline bolt. More particularly, the invention relates to a nanocrystalline bolt having high strength and high toughness and being able to be manufactured without need for heat treatment, as well as to a method for manufacturing the nanocrystalline bolt.

2. Description of the Related Art

Conventionally, high-strength bolts having high strength and high toughness for use in, for example, aircraft are manufactured from 13Cr-8Ni stainless steel or the like by the steps of formation of head portions of bolts by heading, heat treatment, machining, and form rolling (as disclosed in, for example, Japanese Patent Application Laid-Open (kokai) No. H05-247593). In this manner, the manufacture of conventional high-strength bolts indispensably involves heat treatment. Accordingly, conventional high-strength bolts are poor in productivity and high in manufacturing cost.

SUMMARY OF THE INVENTION

An object of the present invention is to solve the above-mentioned problems in the conventional high-strength bolt and in the method for manufacturing the conventional high-strength bolt and to provide a high-strength bolt (nanocrystalline bolt) that can be manufactured without involvement of heat treatment and can exhibit high strength and high toughness equivalent to those of a high-strength bolt manufactured from an alloy steel with involvement of heat treatment, and thus can be manufactured with high productivity and low cost, as well as to provide a method for manufacturing the high-strength bolt (nanocrystalline bolt).

To achieve the above object, the present invention provides a nanocrystalline bolt manufactured by compacting powder formed of nanocrystalline metal particles having a nanometer-scale grain size, extruding a resultant compact so as to obtain a rod stock, and forming the rod stock into a bolt shape without involvement of heat treatment.

According to the present invention, a rod stock (nanocrystalline material) used to form the nanocrystalline bolt is formed by extruding a compact that is formed by compacting powder formed of naoncrystalline metal particles having a nanometer-scale grain size. The rod stock itself has high strength and high toughness. Therefore, without involvement of heat treatment, by merely subjecting the rod stock to a conventional bolt-forming process (heading, machining, and form rolling), a yielded high-strength bolt; i.e., a nanocrystalline bolt, exhibits strength and toughness equivalent to those of a high-strength bolt that is manufactured from an alloy steel with involvement of heat treatment. In the case where the nanocrystalline bolt assumes the form of a headed bolt having a long length, manufacture thereof does not require a corrective straightening process effected by heat treatment. Thus, the high-strength bolt can be manufactured with high productivity and low cost.

Preferably, the metal powder used in the present invention is of stainless steel.

By virtue of the above feature, a nanocrystalline bolt, which is a high-strength bolt having excellent corrosion resistance, high toughness, and high strength, can be readily manufactured from a general material. Particularly, the nanocrystalline bolt can serve as a high-strength bolt suited for use in a plant where stress-corrosion cracking is highly likely to occur.

The present invention also provides a method for manufacturing a nanocrystalline bolt, comprising the steps of compacting powder formed of nanocrystalline metal particles having a nanometer-scale grain size, extruding a resultant compact so as to obtain a rod stock, and forming the rod stock into a bolt shape without involvement of heat treatment.

According to the method of the present invention, a rod stock (nanocrystalline material) used to form the nanocrystalline bolt is formed by extruding a compact that is formed by compacting powder formed of naoncrystalline metal particles having a nanometer-scale grain size. The rod stock itself has high strength and high toughness. Therefore, without involvement of heat treatment, by merely subjecting the rod stock to a conventional bolt-forming process (heading, machining, and form rolling), there can be manufactured a nanocrystalline bolt; i.e., a high-strength bolt that exhibits strength and toughness equivalent to those of a high-strength bolt that is manufactured from an alloy steel with involvement of heat treatment. In the case where the nanocrystalline bolt assumes the form of a headed bolt having a long length, the method does not involve a corrective straightening process effected by heat treatment. Thus, the high-strength bolt can be manufactured with high productivity and low cost.

Preferably, the metal powder used in the method of the present invention is of stainless steel.

By virtue of the above feature, the method of the present invention can readily manufacture, from a general material, a nanocrystalline bolt, which is a high-strength bolt having excellent corrosion resistance, high toughness, and high strength. Particularly, the method can manufacture a high-strength bolt suited for use in a plant where stress-corrosion cracking is highly likely to occur.

BRIEF DESCRIPTION OF THE DRAWINGS

Various other objects, features and many of the attendant advantages of the present invention will be readily appreciated as the same becomes better understood by reference to the following detailed description of the preferred embodiment when considered in connection with the accompanying drawings, in which:

FIG. 1 is a side view of a nanocrystalline bolt according to an embodiment of the present invention;

FIG. 2 is a side view of the nanocrystalline bolt as viewed after subjection to a tensile test;

FIG. 3 is a side view of the nanocrystalline bolt as viewed after subjection to a bifacial shearing test;

FIG. 4 is a side view of the nanocrystalline bolt as viewed after subjection to a fatigue test; and

FIG. 5 is a table comparing the results of the tensile and shearing tests conducted on the test samples of the embodiment with corresponding values of a standard material.

DESCRIPTION OF THE PREFERRED EMBODIMENT

An embodiment of the present invention will next be described in detail with reference to the drawings.

A stainless steel equivalent to JIS SUS410 is used to make a nanocrystalline material. The stainless steel is pulverized into fine particles having a particle size of tens of μm. Next, the particles are further pulverized to a nano-pulverizer, such as a medium-stirring mill, to thereby be formed into finer particles having a nanometer-scale (nm-scale) particle size.

The powder having a nanometer-scale particle size is compacted. The resultant compact is hot-extruded into a nanocrystalline stainless steel rod stock (nanocrystalline material). In the course of the hot extrusion, Zr contained in the powder material suppresses grain growth, thereby controlling the grains size to a value on a predetermined order. Such a nanocrystalline powder is supplied by Japan Ultra-high Temperature Materials Research Center.

Next, the rod stock is cut into pieces each having a predetermined length. Each of the cut pieces is subjected to a conventional bolt-forming process (heading, machining, and form rolling) to thereby be finished to a bolt (nanocrystalline bolt) 1 as shown in FIG. 1. In the bolt 1, the metallographic grain size is controlled to a nanometer scale. Heat treatment is not performed before and after the bolt-forming process.

EXAMPLE

The bolts 1 of the present embodiment were manufactured as test samples from the above-mentioned rod stock having a diameter of 5 mm. Each of the test sample bolts 1 has a head portion 2 and a long bolt body portion 3. A threaded portion 3 a is formed on the bolt body portion 3 while extending along a predetermined length from an end portion of the bolt body portion 3. The test sample bolts 1 have the dimensions shown in FIG. 1. The threaded portion 3 a has threads of #10-32UNJF-3A. The test sample bolts 1 were subjected to the tensile, shearing, and fatigue tests.

The tensile test was conducted in accordance with American Standard MIL-STD-1312 TEST No. 8. The threaded portion 3 a was ruptured at a load of 17.50 kN (see FIG. 2).

The shearing test was conducted in accordance with American Standard MIL-STD-1312 TEST No. 13, which specifies a bifacial shearing test. Shear fracture occurred at a load of 28.55 kN (see FIG. 3).

The fatigue test was conducted in accordance with American Standard MIL-STD-1312 TEST No. 11. The test was conducted under the following cycle test conditions: fatigue high-load: 40% of tensile break load (7 kN); fatigue low-load: 0.1% of fatigue high-load (0.7 kN); and frequency: 30 Hz. Thread rupture occurred at 3,994,900 cycles (see FIG. 4).

Since no criteria were available for the above test results, the test results were compared with corresponding values of JIS SUS410 serving as a standard material.

JIS G4303 SUS410 has a tensile strength of at least 540 N/mm² as measured in a quenched and tempered condition. A tensile strength of the standard material as reduced to an effective cross-sectional area of 12.73 mm² of the threaded portion 3 a of the test sample bolt 1 is calculated as 540 N/mm²×12.73 mm²=6.88 kN.

A shearing strength of the standard material as reduced to a bolt diameter of 4.801 mm of the test sample bolt 1 is calculated as 540 N/mm²×0.6×18.09 mm²=5.87 kN. The bifacial shearing strength is two times of 5.87 kN; i.e., 11.74 kN.

The above results are shown in the TABLE 1.

As is apparent from the TABLE 1, the nanocrystalline bolt 1 of the present embodiment, which is manufactured from a stainless steel equivalent to SUS410, has a tensile strength and a shearing strength that are two times or more of those of the standard material JIS SUS410.

The above-mentioned high-strength feature is attained because the rod stock (nanocrystalline material), which is manufactured by compacting a nanocrystalline powder of stainless steel and extruding the resultant compact, itself has high strength and high toughness. Furthermore, since the nanocrystalline bolt 1 is manufactured from a stainless steel, the nanocrystalline bolt 1 has excellent corrosion resistance and particularly can serve as a high-strength bolt suited for use in a plant where stress-corrosion cracking is highly likely to occur.

Even when the nanocrystalline bolt (high-strength bolt) 1 assumes the form of a headed bolt having a long length, the method for manufacturing the nanocrystalline bolt 1 does not need to involve a corrective straightening process effected by heat treatment.

Since the manufacturing process does not involve heat treatment as mentioned above, the nanocrystalline bolt (high-strength bolt) 1 can be manufactured with high productivity and low cost.

The test sample bolts used in the above tests were manufactured by subjecting a rod stock of nanocrystalline stainless steel to a bolt-forming process (heading, machining, and form rolling) without involvement of heat treatment. Accordingly, the test revealed that the test sample bolts were equivalent in strength to heat-treated bolts used widely in general aircraft and having a tensile strength of 1,240 MPa (180 KSI (Kp/in²)).

The present invention is not limited to the above embodiment, but may be embodied in various other forms without departing from the scope of the invention.

For example, the metal material is not limited to JIS SUS410. Other stainless steels, such as precipitation-hardening-type 13Cr-8Ni-2Mo, may be used as appropriate. 

1. A nanocrystalline bolt manufactured by compacting powder formed of nanocrystalline metal particles having a nanometer-scale grain size, extruding a resultant compact so as to obtain a rod stock, and forming the rod stock into a bolt shape without involvement of heat treatment.
 2. A nanocrystalline bolt according to claim 1, wherein the metal powder is of stainless steel.
 3. A method for manufacturing a nanocrystalline bolt, comprising the steps of: compacting powder formed of nanocrystalline metal particles having a nanometer-scale grain size; extruding a resultant compact so as to obtain a rod stock; and forming the rod stock into a bolt shape without involvement of heat treatment.
 4. A method for manufacturing a nanocrystalline bolt according to claim 3, wherein the metal powder is of stainless steel. 